1. Field of the Invention
The present invention relates generally to a power supply circuit having a supplemental power source which is intermittently used and which is brought into service each time a main power source is coupled to a high power drain circuit. More specifically, the present invention relates to a power supply circuit wherein the supplemental power source is switched to a circuit, which is susceptible to power drop, in response to switching over of a main power source to a high power drain circuit. The present invention has found extensive use in a power supply circuit of a TDMA (time division multiple access) mobile unit.
2. Description of the Related Art
While the present invention is described in connection with a TDMA mobile radio unit, it will be understood that the present invention may also be applied to a device wherein a high power drain circuit is cyclically energized and induces a relatively large power drop which tends to adversely affect the operation of other circuit(s).
TDMA techniques, in addition to the current extensive use thereof in a satellite communications system, have recently assumed an important role in a mobile digital communications system.
As is well known in the art, in a TDMA mobile communications system, a plurality of mobile units located within a given service area are respectively allowed to communicate with an associated land site using allocated time slots of each consecutive TDMA frame. The time slot used by a given mobile unit has therefore the same period as the TDMA frame. That is, the TDMA mobile unit periodically transmits information within the allocated time slots in short bursts instead of continuously.
Prior to turning to the present invention it is deemed preferable to discuss, with reference to FIG. 1, a known power supply circuit of a TDMA mobile unit to which the present invention is preferably applicable.
A TDMA mobile unit per se is well known in the art and therefore, the overall block diagram thereof is not discussed in detail merely for the purpose of simplifying the discussion. Further, since the present invention is directed to improvements of a power supply circuit, the present invention can readily be understood without reference to signals or data which are used in the TDMA mobile unit and thus, they are omitted from the accompanying drawings for the sake of simplicity.
As shown in FIG. 1, a power source (primary or secondary battery) 10 is coupled so as to supply electric power to an output power amplifier 12 and a frequency synthesizer 14 both of which are arranged in parallel with the battery 10. The battery 10 has its internal resistance depicted by Rin. The power amplifier 12 is to amplify an outgoing signal from the mobile unit and thus consumes a considerable amount of power. Although not shown in FIG. 1, the battery 10 is also coupled to other circuits of the TDMA mobile unit for the purpose of suitable power supply thereto.
A switch 16 is provided between a power line 18 and the output power amplifier 12 in order to implement battery power conservation. More specifically, as mentioned above, the TDMA mobile unit intermittently communicates with an land site using allocated time slots in each consecutive TDMA frame. Therefore, in view of high power drain at the final power amplifier 12, it is highly desirable to supply power thereto only when the power supply is required. To this end, the switch 16 is selectively turned on and off in response to a TDMA transmission timing control signal (depicted by CS) from a controller (not shown).
However, while the power amplifier 12 is energized via the switch 16 which has been closed, a power voltage drop at the line 18 exhibits a relatively large value. This voltage drop tends to undesirably change a reference frequency produced from the frequency synthesizer 14.
The frequency synthesizer 14 is usually configured so as to provide the reference frequency accurate to the required stability over power supply variations. However, it is highly desirable to effectively suppress the power voltage drop in view of its synergistic effects with ambient temperature changes (for example).
For a better understanding of the problem of the above mentioned known power supply circuit, a particular example is given below.
It is assumed that: (a) the voltage of the battery 10 is 4.8V, (b) the internal resistance Rin is 0.5.OMEGA. and (c) the current flowing through the power amplifier 12 is 2A. In this instance, when the power amplifier 12 is energized, the voltage appearing at the power line becomes 3.8V (=4.8-0.5.times.2). The power voltage is therefore lowered by one-volt, which when applied to the frequency synthesizer 14, is liable to undesirably change the reference frequency.
On the other hand, Japanese Laid-open Patent Application No. 62-250876 discloses a supplemental battery which is coupled in parallel with an electrolytic condenser and which is used to compensate for accidental voltage drop or instantaneous power failure on a commercial 3-phase ac (alternate current) power line. That is, the supplemental battery is to compensate for the accidental power drop which adversely affects sequential operations of a voltage invertor which is provided upstream of an ac load such as a 3-phase motor. This related art is not directed to a power supply circuit wherein a supplemental power source is intermittently switched to a circuit susceptible to power drop every time a main power battery supplies power to a high power drain circuit.
It is therefore desired to prevent the intermittently occurring instantaneous potential drop from being supplied to a circuit which is susceptible to such a reduction.